Conventionally, in a magnetic resonance imaging (MRI) system superconducting magnets are used to generate one or more magnetic fields, e.g., a main magnetic field, a gradient magnetic field, or a shielding magnetic field. Because of the characteristics of the superconducting material, most superconducting magnets need to work at an extremely low temperature for providing a stable magnetic field. For example, the operation temperature of superconducting magnet made by NbTi is approximately 4.2K, and the operation temperature of superconducting magnet made by MgB2 is approximately 20-30K. Usually a liquid cryogen, such as liquid nitrogen, liquid helium, liquid hydrogen and liquid neon, may be used to provide such a low temperature environment. Such a liquid cryogen may be continuously consumed during the transportation of an MRI system from a manufacturer to a hospital or during the operation of the MRI system, and thus the cryogen may need to be re-filled regularly or from time to time.
The volume of a liquid cryogen needed in an MRI system may at least partially depend on the size of system cavities housing the liquid cryogen. Since the liquid cryogen, e.g., liquid helium, may be expensive, there is a need for a more efficient and cost-efficient way to control or adjust the size of such system cavities and improve the cooling efficiency, in order to reduce the consumption of such an expensive cryogen.